Magma circulating type crystallizer with special impeller



' April 1968 TATUSI KASAI 3,378,236

MAGMA CI RCULATING TYPE CRYSTAL-LIZER WITH SPECIAL IMPELLER Filed Oct. 27. 1966 United States Patent 0 3,378,236 MAGMA ClRCULATlNG TYPE CRYSTALLIZER WITH SPECIAL IMPELLER Tatusi Kasai, Yokohama, Japan, assignor to Tsukishima Kikai (30., Ltd, Tokyo, Japan, a corporation of Japan Filed Get. 27, 1966, Ser. No. 589,890 Claims priority, application Japan, Oct. 27, 1965, 46/ 65,47 9 6 Claims. (Cl. 259-95) ABSTRACT OF THE DISCLOSURE A slurry circulation type crystallizer including a rotating draft tube driven by a vertical drive shaft, said draft tube having an external rotor to induce a downward flow of slurry outside the draft tube and an internal rotor to induce an upward flow of slurry within the draft tube.

The present invention relates to a magma circulation mechanism which can obtain crystals having unworn, smooth and lustrous surfaces and uniform in crystal size by preventing spalling of the crystals in the course of agitation when the crystals are educed or grown in a magma circulation type crystal growing apparatus.

In a conventional magma or slurry circulation type crystallizer, an agitating rotor is provided within a draft tube disposed at the center portion of the tank, and the agitating rotor is rotationally driven so as to make the magma or slurry which contains crystals and is apt to descend in the liquid within the tank to circulate so that substantial size of crystals can be obtained.

However, in the prior apparatus of the type as described above, since the agitating rotor is employed so as to make the slurry or magma descending in the draft tube to ascend so that desired circulation is obtained, the rotor is restricted in the shape and the size by those of the tank and the draft tube construction. Further, the agitating rotor must be driven at a very high speed because the atcending speed of the liquid is required to be greater than the descending speed of the crystals themselves in order to make them uniform circulation. Therefore, the

eripheral speed of the agitating rotor becomes so high that the crystals collide with the rotor blade and result in spalling and wearing of the crystals. Moreover, when the agitating rotor operates to move the liquid upwardly, the crystals which move downwardly relative to the liquid are precipitated on the rotor blades. This causes wear of the blade surfaces as well as wear of the crystals themselves, and therefore the crystals get rough surfaces so that their luster is lost. Further, since the edges of the crystals are rounded, their growth is suppressed. Thus, the commercial grade of the crystals are substantially lowered. The fine crystals created by the surface wear of the crystals have to be removed from the apparatus or dissolved, otherwise the crystals of desired size cannot be obtained. Because, the number of the final products is equal to the total number of the crystallites and the fine crystals subtracted by the number of the crystals which are dissolved or removed from the apparatus, and if the crystallites and the fine crystals are greater in number, the distribution range of the size of the crystals becomes greater, so that fine crystals will be included in the final products. When it is desired to obtain uniform products, the size of the crystals have to be suppressed to smaller size as a whole.

Further, the conventional apparatus does not have size control action in the tank and contains uniform slurry, so that it has been difficult to obtain products of uniform crystal size. In order to overcome this difliculty, the conventional apparatus has to be provided with means for 3,378,236 Patented Apr. 16, 1968 ICC controlling crystal size at the outside of the apparatus.

An object of the present invention is to eliminate the above described disadvantages of the conventional apparatus.

Another object of the present invention is to reduce the rotation speed of the agitating rotor to substantially decrease the peripheral speed of the rotor blade so that the wear of the crystals due to abrasing contact with the rotor blade is avoided and that high. grade and lustrous crystals are obtained.

Further object of the present invention is to reduce the dimension of the apparatus by preventing the creation of the fine crystals, wear of the crystals, and decreasing of the growing speed of the crystals, so that economical crystal growth is obtained.

The present invention will now be described with regard to preferred embodiments taking reference to the accompanying drawings, in which;

l lG'URE l is a longitudinal section illustrating an embodiment of the magma circulation type crystallizer in accordance with the present invention;

FIGURE 2 is a cross-sectional view taken substantially along the line A-A of FIGURE 1;

FIGURE 3 is a longitudinal section similar to FIG- DRE 1 but showing a further embodiment of the present invention;

FIGURE 4 is a cross-sectional view substantially taken along the line B-B of FIGURE 3;

FlGURE 5a is a plan view of a small agitating rotor; and

FIGURE 51) is also a plan view of a large agitating rotor which is two times in diameter as compared with the rotor shown in FIGURE 5a.

Referring now to the drawings, more particularly to FIGURES 1 and 2, the liquid is supplied through the liquid supply port 1 and passes upwardly through the lower draft tube 2 into the upper draft tube 3. The liquid further passes upwardly through the spaces provided between the inner rotor blade disposed within the upper draft chamber 3, and during this period, it is mixed with the slurry within the tank. When the mixture comes to such a depth Where the pressure of the mixture becomes equal to the pressure within the vaporizing chamber 5 added by the liquid pressure, it comes to a boil within the upper draft tube 3 and arrives at the evaporating surface 6 where it is completely evaporated. In this i11- stancc, the solute which had been previously dissolved in the solvent that is already evaporated is dissolved in a supersaturated condition in the solution which is circulating in the tank under the operation of the inner rotor 4. Therefore, the crystals which are floating in this supersaturated solution occlude the supersaturated solute, and thus there occurs growth of the crystals. At the outside of the upper draft tube 3, there is. provided an outer rotor 7 which induces the downwardly directed flow of the liquid along the outside of the upper draft tube 3. The crystals fall within this flow at the speed faster than the speed of the downwardly directed flow. Since the crystals move downwardly with reference to the downwardly directed How, they continuously contact with fresh supersaturated solution so that they are continuously grown and. thereafter sucked from the lower end of the draft tube 2 to effect repeated circulation.

When the crystals are grown to a predetermined size, they are separated from the circulating flow and move along the conical portion 8 into the classifying leg portion 9. At the lower end of the classifying leg portion 9, the crystals which are greater than a predetermined size are taken out by means of clean liquid through the discharge port 10 as the product slurry and the smaller crystals are returned to the circulating flow in order to obtain further growth of crystals. The liquid from which zhe crystals had been thus educed are reheated or slowly moved upwardly along the outer side of the baille plate 11 to take out from the apparatus. During this upward movement, the crystals are precipitated and the liquid is discharged through the discharge port 12. as the clean liquid. The evaporated solvent is completely separated from liquid droplets in the evaporating chamber 5 and is discharged in a vapor state through the vapor outlet port 13 into atmosphere, vacuum condenser and the like. In the drawings, the numeral 14 shows a drive shaft which is driven by a motor 15, and the numeral 16 is a support for carrying the lower draft tube 2 at its one end, the other end of said support 16 being secured to the bafile plate 31.

According to the circulating method of the present invention, the circulating flow is formed by the outer rotor disposed at the outside of the upper draft tube, and the inner rotor merely assists the upwardly directed flow within the upper draft tube when the outer rotor is driven so as to compensate the resistance of the support of the lower draft tube and prevent the slurry from being spalled. In other words, since the inner rotor is so arranged that it does not positively accelerate the slurry upwardly and does not interfere with the flow or" the slurry, the abrasing contact between the inner rotor blades and the crystals are substantially reduced.

When the liquid is circulated by means of the uter rotor, the liquid flow along the outside of the upper and lower draft tubes is downwardly directed and the crystals floating in the liquid fall faster than the descending liquid so that, even if the outer rotor is driven, the crystals do not ride on the surfaces of the blades of the outer rotor and the abrasing contact between the blades and the crystals are substantially eliminated. The crystals obtained by the apparatus of the present invention are free from wear and keep their shapes, and according to the present invention, since the edges of the crystals are not rounded, their growing rate is substantially high. Further, the present invention is free from creation of line crystals due to the wear or spalling of the crystals which has been encountered in a conventional apparatus wherein a small rotor is driven at a high speed in order to invert the direction of movement of the descending crystals t move upwardly, so that the apparatus rate of growth of the whole crystals becomes very high and the whole dimension of the apparatus can be reduced.

In the apparatus of the present invention, few crystals are spalled at the tip of each blade of the agitating rotor. In order to obtain substantially stable supersaturation of a given material, the minimum circulation has to be maintained. With regard to two different agitating rotors having diameter ratio of 1:2, the speeds of rotation required for providing the minimum circulation are compared by the fOllOWing equations.

In order to give the same circulation by the two rotors,

the following relation must be maintained 1rD -P N 1r(2D) P N 4 'T where D is the diameter of the smaller agitating rotor;

P is the pitch of the smaller agitating rotor;

P is the pitch of the larger agitating rotor;

N is the tip speed of the smaller agitating rotor; and N is the tip speed of the larger agitating rotor.

accordingly,

This means that the larger agitating rotor is required to be rotationally driven at a speed of only one eighth of the speed of the smaller agitating rotor.

The tip speeds (V and (V are expressed as follows:

Thus, the larger agitating rotor is required to be driven at the tip speed of one fourth of that of the smalle agitating rotor.

It will be seen from the above equations that, by using the larger agitating rotor, the spalling of the crystals can be remarkably reduced.

In a crystallizing apparatus, the rate of creation of the crystallites relates to the Reynolds number of the apparatus. The rate of creation of the crystallites increases as the Reynolds number increases. The Reynolds number is defined by the fohowing equation.

where;

NRe is the Reyonlds number;

p is the specific weight of the liquid; and ,n is the viscosity of the liquid.

Comparing the two different size of agitating rotor having diameter ratio of 2:1 in respect of Reynolds numher,

for smaller rotor:

for larger rotor;

replacing N by the Equation 1,

Thus, it will be seen that in the Reynolds number of the larger agitating rotor is equal to one half of that of the smaller agitating rotor.

Accordingly, by using the larger agitating rotor, the rate of creation of the crystallites is remarkably reduced so that the larger size of crystals can be obtained.

FIGURES 3 and 4 show another embodiment of the present invention. In this example, the draft tube assembly is composed of three parts, i.e. the lower, intermediate and upper draft tubes 21, 22 and 23 respectively, the lower draft tube 21 being smaller in diameter than the intermediate and the upper draft tubes 22 and 2.3. Therefore, the quantity of the circulating liquid accelerated by the outer agitating rotor 7 may be greater than that of the ascending flow through the lower draft tube 21, the balance of the liquid being passed upwardly through the gap formed between the draft tubes 21 and 22. The smaller crystals are apt to be carried by the flow passing through the gap between the draft tubes 21 and 22, while the larger crystals are apt to fall along the outside of the lower draft tube 21 and thus the larger crystals are collected in the neighbourhood of the product slurry take out port 10. Thus, in this type of apparatus, crystals of uniform size can automatically be selected. In this example, the lower agitating rotor 24 are only supplementarily used and this may be omitted without any inconveniences.

As obvious from the above description, by employing the present invention, the wear of the crystals can be reduced and the crystals retain their natural shapes. Thus, high quality and commercially valuable crystals having lustrous surfaces and free from surface impurity can be successfully obtained. Further, according to the present invention, the creation of fine crystals due to abrasing contact and the spalling of the crystals at the tips of the blades of the agitating rotor can be remarkably reduced, and since the rotational speed of the agitating rotor is low, creation of the crystallites is so reduced that the crystals of larger and uniform size can be successfully obtained. Moreover, by modifying the draft tube assembly, crystals of uniform size can be selected so that the product becomes uniform in size.

What is claimed is:

1. In a slurry circulation type crystallizer, a vertical drive shaft, a rotating draft tube driven by said shaft, 21 first rotor mounted on the external surface of said draft tube and having blades pitched to induce a downward flow of said slurry outside of said draft tube, and a second rotor mounted on the internal surface of said draft tube and having blades pitched to induce an upward flow of said slurry within said draft tube.

2. The slurry circulation type crystallizer of claim 1 containing also a fixed draft tube mounted beneath said rotating draft tube.

3. The slurry circulation type crystallizer of claim 2 wherein said fixed draft tube has a diameter less than that of said rotating draft tube.

4. A slurry circulation type crystallizer of claim 3 wherein a portion of said fixed draft tube extends upwardly within said rotating draft tube.

5. The slurry circulation type crystallizer of claim 4 containing also a third rotor mounted on said drive shaft and having blades pitched to induce an upward flow of said slurry within said fixed draft tube.

6. The slurry circulation type crystallizer of claim 2 containing also a fixed draft tube mounted above said rotating draft tube.

References Cited FOREIGN PATENTS 12/1964 France.

9/1956 Great Britain. 

